Typically, prior art feedwater control systems for nuclear power plants use a combination of feedwater pumps, control valves and steam generators. The feedwater pumps are usually connected in parallel and then each of these pumps is fluidically connected to a control valve which, in turn, is fluidically connected to a steam generator. In these systems, each of the control valves can be manually or automatically operated to obtain the demanded flow of feedwater to its respective steam generator. In this approach, the feedwater pumps can also be manually or automatically operated to minimize the pressure drops across the control valves. The primary problem with such a system is that if a control valve fails in the wide open position, the feedwater pumps speed up in an attempt to control the pressure drop across the failed valve. This results in excessive water being dumped into the steam generator which causes overcooling of the primary system if corrective measures are not taken.
Another problem that arises in nuclear power plants is that most physical phenomena which occur therein are only understood intuitively and arise out of a complex set of variables or conditions, and thus the resulting corrective measures might not be appropriate. For example, process structure changes resulting from pumps tripping, valves failing locked, etc., can result in different feedwater control requirements. Typically, the foregoing conditions are corrected by the operator of the system who uses his experience and intuition as to what remedial measures have to be taken. These remedial measures might not be proper and might result in further system instabilities.
Because of the foregoing, it has become desirable to develop a control system which regulates and controls the operation of a nuclear power plant feedwater system under a multiplicity of operational modes.